I. Field of the Invention
This invention relates generally to spread spectrum communication systems and, more particularly, to an RF signal repeater.
II. Description of the Related Art
In a wireless telephone communication system, many users communicate over a wireless channel to connect to wireline telephone systems. Communication over the wireless channel can be one of a variety of multiple access techniques which facilitate a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). The CDMA technique has many advantages and an exemplary CDMA system is described in U.S. Pat. No. 4,901,307 issued Feb. 13, 1990 to K. Gilhousen et al., entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of the present invention and incorporated herein by reference.
In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations using CDMA spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity.
The CDMA modulation techniques disclosed in the '307 patent offer many advantages over narrow band modulation techniques used in communication systems using satellite or terrestrial channels. The terrestrial channel poses special problems to any communication system particularly with respect to multipath signals. The use of CDMA techniques permits the special problems of the terrestrial channel to be overcome by mitigating the adverse effect of multipath, e.g. fading, while also exploiting the advantages thereof.
In a CDMA cellular telephone system, the same frequency band can be used for communication in all base stations. At the receiver, separable multipath, such as a line of site path and another one reflecting off of a building, can be diversity combined for enhanced modem performance. The CDMA waveform properties provide processing gain that is used to discriminate between signals that occupy the same frequency band. The high speed pseudonoise (PN) modulation allows many different propagation paths of the same signal to be separated, provided the difference in path delays exceeds the PN chip duration. If a PN chip rate of approximately 1 MHz is employed in a CDMA system, the full spread spectrum processing gain, equal to the ratio of the spread bandwidth to the system data rate, can be employed against paths having delays that differ by more than one microsecond. A one microsecond path delay differential corresponds to differential path distance of approximately 300 meters. The urban environment typically provides differential path delays in excess of one microsecond.
The multipath characteristic of a channel can result in signal fading. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors are added destructively, yielding a received signal that is smaller than either individual vector. For example, if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of .chi. dB, a time delay of .delta. with a phase shift of .THETA. radians, and the second path has an attenuation factor of .chi. dB, a time delay of .delta. with a phase shift of .THETA.+.pi. radians, no signal would be received at the output of the channel.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for base station and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", issued Oct. 8, 1991, also assigned to the assignee of the present invention.
In the CDMA cellular system described in the above-referenced '307 patent, each base station provides coverage to a limited geographic area and links the mobile units in its coverage area through a cellular system switch to the public switched telephone network (PSTN). When a mobile unit moves to the coverage area of a new base station, the routing of that user's call is transferred to the new base station. The base station-to-mobile unit signal transmission path is referred to as the forward link and the mobile unit-to-base station signal transmission path is referred to as the reverse link.
As described above, the PN chip interval defines the minimum separation two paths must have in order to be combined. Before the distinct paths can be demodulated, the relative arrival times (or offsets) of the paths in the received signal must first be determined. A channel element modem performs this function by "searching" through a sequence of potential path offsets and measuring the energy received at each potential path offset. If the energy associated with a potential offset exceeds a certain threshold, a demodulation element may be assigned to that offset. After demodulating the signal present at that path offset can then be summed with the contributions of other demodulation elements at their respective offsets. A method and apparatus of demodulation element assignment based on searcher element energy levels is disclosed in co-pending U.S. patent application Ser. No. 08/144,902 entitled "DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS," filed Oct. 28, 1993, assigned to the assignee of the present invention. Such a diversity or rake receiver provides for a robust digital link, because all paths have to fade together before the combined signal is significantly degraded.
In a cellular or personal communication telephone system, maximizing the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is extremely important. System capacity in a spread spectrum system can be maximized if the transmit power of each mobile unit is controlled such that each transmitted signal arrives at the base station receiver at the same level. In an actual system, each mobile unit may transmit the minimum signal level that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal transmitted by a mobile unit arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to interference from the other mobile units. On the other hand, if the mobile unit transmitted signal is at a power level that is too high when received at the base station, communication with this particular mobile unit is acceptable but this high power signal acts as interference to other mobile units. This interference may adversely affect communications with other mobile units.
Therefore to maximize capacity in an exemplary CDMA spread spectrum system, the transmit power of each mobile unit in communication with a base station is controlled by the base station to produce the same nominal received signal power at the base station. In the ideal case, the total signal power received at the base station is equal to the nominal power received from each mobile unit multiplied by the number of mobile units transmitting within the coverage area of the base station plus the power received at the base station from mobile units in the coverage areas of neighboring base stations.
The path loss in the radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the mobile unit, operates on a different frequency than the reverse link, from the mobile unit to the base station. However because the forward link and reverse link frequencies are within the same frequency band, a significant correlation exists between the average path loss of the two links. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time. However, the characteristics of the fading on the channel are the same for both the forward and reverse link because the frequencies are within the same band. Therefore the average of fading over time for both links is typically the same.
In an exemplary CDMA system, each mobile unit estimates the path loss of the forward link based on the total power at the input to the mobile unit. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the mobile unit. From the estimate of the average forward link path loss, the mobile unit sets the transmit level of the reverse link signal.
Mobile unit transmit power is also controlled by one or more base stations. Each base station with which the mobile unit is in communication measures the received signal strength from the mobile unit. The measured signal strength is compared to a desired signal strength level for that particular mobile unit at that base station. A power adjustment command is generated by each base station and sent to the mobile unit on the forward link. In response to the base station power adjustment commands, the mobile unit increases or decreases the mobile unit transmit power by a predetermined amount.
Various methods exist for switching the mobile unit from one base station to another (known as "handoff"). One such method is termed a "soft" handoff, in which communication between the mobile unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This method is considered a soft handoff in that communication with the subsequent base station is established before terminating communication with the original base station. When the mobile unit is communicating with two base stations, a single signal for the end user is created from the signals from each base station by a cellular or personal communication system controller. U.S. Pat. No. 5,267,261 which is incorporated by this reference and assigned to the assignee of the present invention, discloses a method and system for providing communication with the mobile unit through more than one base station during the handoff process, i.e., providing soft handoff.
When a mobile unit is in communication with more than one base station, power adjustment commands are provided from each base station. The mobile unit acts upon these multiple base station power adjustment commands to avoid transmit power levels that may adversely interfere with other mobile unit communications and yet provide sufficient power to support communication from the mobile unit to at least one of the base stations. This power control mechanism is accomplished by having the mobile unit increase its transmit signal level only if every base station with which the mobile unit is in communication requests an increase in power level. The mobile unit decreases its transmit signal level if any base station with which the mobile unit is in communication requests that the power be decreased. A system for base station and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109 as noted above. Further information for a system of base station and mobile unit power control is disclosed in U.S. Pat. No. 5,265,199 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", issued Nov. 23, 1993, also assigned to the assignee of the present invention.
Base station diversity at the mobile unit is an important consideration in the soft handoff process. The power control method described above operates optimally when the mobile unit communicates with each base station through which communication is possible. In doing so, the mobile unit avoids inadvertently interfering with communications through a base station receiving the mobile unit's signal at an excessive level but unable to communicate a power adjustment command to the mobile unit because communication is not established therewith.
It is also desirable to control the relative power used in each data signal transmitted by the base station in response to control information transmitted by each remote unit. The primary reason for providing such control is to accommodate the fact that in certain locations the forward channel link may be unusually disadvantaged. Unless the power being transmitted to the disadvantaged remote unit is increased, the signal quality may become unacceptable. An example of such a location is a point where the path loss to one or two neighboring base stations is nearly the same as the path loss to the base station communicating with the remote unit. In such a location, the total interference would be increased by three times over the interference seen by a remote unit at a point relatively close to its base station. In addition, the interference coming from the neighboring base stations does not fade in unison with the signal from the active base station as would be the case for interference coming from the active base station. A remote unit in such a situation may require 3 to 4 dB additional signal power from the active base station to achieve adequate performance.
At other times, the remote unit may be located where the signal-to-interference ratio is unusually good. In such a case, the base station could transmit the desired signal using a lower than normal transmitter power, reducing interference to other signals being transmitted by the system.
To achieve the above objectives, a signal-to-interference measurement capability can be provided within the mobile unit receiver. This measurement is performed by comparing the power of the desired signal to the total interference and noise power. If the measured ratio is less than a predetermined value, the mobile unit transmits a request to the base station for additional power on the forward link signal. If the ratio exceeds the predetermined value, the mobile unit transmits a request for power reduction. One method by which the remote unit receiver can monitor signal-to-interference ratios is by monitoring the frame error rate (FER) of the resulting signal.
The base station receives the power adjustment requests from each mobile unit and responds by adjusting the power allocated to the corresponding forward link signal by a predetermined amount. The adjustment would usually be small, typically on the order of 0.5 to 1.0 dB, or around 12%. The rate of change of power may be somewhat slower than that used for the reverse link, perhaps once per second. In the preferred embodiment, the dynamic range of the adjustment is typically limited such as from 4 dB less than nominal to about 6 dB greater than nominal transmit power.
All the cellular radiotelephone systems operate by placing base stations throughout a geographic region such that each base station operates to provide communication with mobile units located within the limited geographic coverage area of the base station. With the initial deployment of the CDMA system, the CDMA system must work in areas currently covered by AMPS or TDMA systems where the two systems overlap. The AMPS and TDMA base station locations and corresponding coverage areas may be separate and distinct from the CDMA base stations and coverage areas. Likewise, within a particular technology system (AMPS, CDMA, or TDMA), there are generally two competing service providers within a given area typically referred to as the A and B carriers. These service providers often choose different base station locations from their competitor. In each of these situations, a mobile unit communicating using a first carrier or technology, might be far away from the base station with which it is in communication while being close to another base station with which it does not communicate. In such a situation, the desired receive signal is weak in the presence of strong multi-tone interference which can cause problems for a mobile unit.
The multi-tone interference encountered by the mobile unit from the narrow-band AMPS or TDMA signals can create distortion within the mobile unit. If the distortion products produce spurs that fall in the CDMA band used by the mobile unit, receiver and demodulator performance can be degraded.
Third-order distortion products occur when two tones are injected in a receiver. For example, if one tone at frequency f.sub.1 at power level P.sub.1 and a second tone at frequency f.sub.2 at power level P.sub.2 is injected into a receiver, third-order distortion products are created at frequencies 2xf.sub.1 -f.sub.2 and 2xf.sub.2 -f.sub.1 at power levels P.sub.12 and P.sub.21 respectively. For example within the cellular band, suppose that CDMA operation is designated from 880 MegaHertz (MHz) to 881.25 MHz. Also suppose that an AMP system operates to provide an FM signal at 881.5 MHz and a second FM signal at 882 MHz. Note that a spurious third order product occurs at 2.times.881.5-882=881 MHz which is directly within the CDMA band.
The power level of the created spurious third order product depends upon the power levels of the two signals which create it and the intermodulation performance of the mobile unit. The amount of distortion generated by the spurious third order product depends on the ratio of the total CDMA power to the total spurious third order product power. Two different means of limiting the distortion caused by the third order products are evident: limit the spurious third order products created by the mobile unit or increase the level of the CDMA signal in relation to the created third order products. Increasing the intermodulation performance of the mobile unit increases the price and power consumption of the mobile unit which is, of course, highly undesirable. A more elegant solution is to increase the CDMA signal level in proximity to the offending base stations.
One method of increasing the signal level of a signal in a given geographic region without providing additional signal generation means is to provide a repeater. A repeater is a device for receiving either one-way or two-way communications signals and delivering corresponding signals which are amplified, reshaped or both. A repeater is used to extend the length, topology or, interconnectivity of the physical medium beyond that imposed by a single segment. A repeater typically receives a signal created by a first usually distant communication unit and retransmits the signal to a second usually distant communication unit where the signal is processed.
One major problem with repeaters is that they tend to be unstable. A repeater can be unstable if it provides large gains to the repeated signal. If the transmitted signal feeds back into the receive portion of the repeater, the repeater can oscillate. If the repeater oscillates it ceases to provide the repeated signal and actually harms the system by providing spurious signals.